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"... Abstract — The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuse ..."

Abstract — The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive softoutput decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in Turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low-complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached. Index Terms — Coded CDMA, instantaneous MMSE filtering, multiuser detection, soft interference cancellation, Turbo processing.

...he asynchronous multiuser detector and the convolutional code, resulting in a prohibitive computational complexity , where is the number of users in the channel, and is the code constraint length. In =-=[7]-=-, some low-complexity receivers which perform multiuser symbol detection and decoding either separately or jointly are studied. Recently iterative (“Turbo”) processing techniques have received conside...

The CDMA channel with randomly and independently chosen spreading sequences accurately models the situation where pseudonoise sequences span many symbol periods. Furthermore, its analysis provides a comparison baseline for CDMA channels with deterministic signature waveforms spanning one symbol period. We analyze the spectral efficiency (total capacity per chip) as a function of the number of users, spreading gain, and signal-to-noise ratio, and we quantify the loss in efficiency relative to an optimally chosen set of signature sequences and relative to multiaccess with no spreading. White Gaussian background noise and equal-power synchronous users are assumed. The following receivers are analyzed: a) optimal joint processing, b) single-user matched filtering, c) decorrelation, and d) MMSE linear processing.

"... A code-division multiple-access system with channel coding may be viewed as a serially-concatenated coded system. In this paper we propose a low complexity method for decoding the resulting inner code (due to the spreading sequence), which allows iterative (turbo) decoding of the serially-concate ..."

A code-division multiple-access system with channel coding may be viewed as a serially-concatenated coded system. In this paper we propose a low complexity method for decoding the resulting inner code (due to the spreading sequence), which allows iterative (turbo) decoding of the serially-concatenated code pair. The per-bit complexity of the proposed decoder increases only linearly with the number of users. Performance within

"... This paper introduces an iterative multiuser receiver for direct sequence code-division multiple access (DS-CDMA) with forward error control (FEC) coding. The receiver is derived from the maximum a posteriori (MAP) criterion for the joint received signal, but uses only single-user decoders. Iteratio ..."

This paper introduces an iterative multiuser receiver for direct sequence code-division multiple access (DS-CDMA) with forward error control (FEC) coding. The receiver is derived from the maximum a posteriori (MAP) criterion for the joint received signal, but uses only single-user decoders. Iterations of the system are used to improve performance, with dramatic effects. Single-user turbo code decoders are utilized as the FEC system and a complexity study is presented. Simulation results show that the performance approaches single-user performance even for moderate signal-to-noise ratios.

"... Abstract—We consider the convergence in norm of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that asymptotically, linear parallel interference cancellation diverges for systems loads of greater than about 17%. Using ..."

Abstract—We consider the convergence in norm of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that asymptotically, linear parallel interference cancellation diverges for systems loads of greater than about 17%. Using known results from the theory of iterative solutions for linear systems we derive optimal or near optimal relaxation parameters for parallel (first– and second-order stationary, Chebyshev) and serial cancellation (successive relaxation) methods. An analytic comparison of the asymptotic convergence factor for the various methods is given. Simulations are used to verify results for finite size systems. Index Terms—CDMA, interference cancellation, iterative methods, multistage receivers, multiuser receivers, random sequences. I.

...salar et al. [27]–[29] is in fact a minor modification of the successive relaxation method discussed in Section III-E. Iterative methods have also been successfully applied to coded multiuser systems =-=[30]-=-–[32]. We restrict our scope to detection of uncoded data. We analyze the convergence properties of several linear iterative implementations of both the decorrelator and the minimum mean squared error...

"... Abstract—Trellis-code multiple-access (TCMA) is a narrowband multiple-access scheme based on trellis-coded modulation. There is no bandwidth expansion, so users occupy the same bandwidth as one single user. The load of the system, in number of bits per channel use, is therefore much higher than the ..."

Abstract—Trellis-code multiple-access (TCMA) is a narrowband multiple-access scheme based on trellis-coded modulation. There is no bandwidth expansion, so users occupy the same bandwidth as one single user. The load of the system, in number of bits per channel use, is therefore much higher than the load in, for example, conventional code-division multiple-access systems. Interleavers are introduced as a new feature to separate the users. This implies that the maximum-likelihood sequence detector (MLSD) is now too complex to implement. Iterative detectors are therefore suggested as an alternative to the joint MLSD. The conventional interference cancellation (IC) detector has lower complexity than the MLSD, but its performance is shown to be far from acceptable. Even after a novel improvement of the IC detector, the performance is unsatisfactory. Instead of using IC, another iterative detector is suggested. This detector updates the branch metric for every iteration, and avoids the standard Gaussian approximation. Simulations show that the performance of this detector can be close to single-user performance, even when the interleaver and the phase offset are the only user-specific features in the TCMA system. Index Terms—Interference cancellation, iterative methods, multiple-access technique, trellis-code multiple-access, trellis-coded modulation. I.

This paper investigates a multiple-access communication receiver system that receives coded data modulated using either direct-sequence code division multiple access or narrowband binary phase shift keying, with an antenna array in a multipath propagation environment. We describe an iterative receiver that improves the initial estimates from the antenna array, and therefore reduces the multiple access interference. Simulation results show that the bit error rate performance approaches that obtained when only one user&apos;s signal is incident on the array. This occurs even with a large number of users in comparison to the product of the spreading gain and array size. Index Terms--- Array signal processing, channel coding, code division multiple access, iterative methods, multiuser channels, turbo codes.

"... We consider the problem of joint decoding and channel estimation for linear multiple-input multipleoutput channels. We assume that the receiver has no apriori knowledge of the channel parameters. The amount of training data required is reduced by using a decision feedback approach to iteratively imp ..."

We consider the problem of joint decoding and channel estimation for linear multiple-input multipleoutput channels. We assume that the receiver has no apriori knowledge of the channel parameters. The amount of training data required is reduced by using a decision feedback approach to iteratively improve channel estimates. We present experimental results for joint channel estimation and decoding of space-time channels. I. Introduction Linear multiple input multiple output (MIMO) channels occur frequently in communications systems. Such channels are of interest whenever simultaneously transmitted signals are non-orthogonal, or are distorted by the channel in such a way that they are correlated at the receiver. The di#erent channel inputs may be statistically independent (for example the various users of a multiple access system), or dependent (for example an OFDM system with inter-carrier interference or a spatial diversity channel). In either case, the optimal receiver will generally...

"... This paper discusses the implementation of a very powerful but computationally efficient, iterative multi-user detector, intended for use as the basestation physical layer receiver for Wideband CDMA. This multiuser detector is unique in that it is fully compliant with the release '99 3GPP stand ..."

This paper discusses the implementation of a very powerful but computationally efficient, iterative multi-user detector, intended for use as the basestation physical layer receiver for Wideband CDMA. This multiuser detector is unique in that it is fully compliant with the release &apos;99 3GPP standard, a result that has not been shown before. The transmitter chain is discussed before the channel model used is introduced. The receiver design approach is then discussed. Special consideration is given to show how channel estimation is integrated into the receiver. We show single cell performance results that indicate a capacity increase of approximately three times over the &quot;conventional&quot; receiver implementation is possible (with and without antenna arrays). Results are then shown using a cell dimensioning simulation tool to indicate the overall gain in terms of increase in cell size. Here we can show that a gain of approximately 50% of the cell size is possible with such a multi-user detection approach. The dimensioning tool is also used to show the increase in capacity for a fixed cell size. Here we can show that a gain in the order of 330% is possible, compared to a conventional receiver approach.